Imaging of biological tissue using photon migration with high directionality techniques

ABSTRACT

A method and system for examination of a subject positioned between input and detection ports of the spectroscopic system applied to the subject. The systems shown include at lease one light source for introducing at one or multiple input ports, electromagnetic non-ionizing radiation of a known time-varying pattern of photon density of a wavelength selected to be scattered and absorbed while migrating in the subject, radiation pattern control means for achieving a directional pattern of emitted resulting radiation that possesses substantial gradient of photon density, at least one detector for detecting the radiation that has migrated in the subject at one or multiple detection ports. The systems also include processing means for processing the detected radiation and creating sets of data, and evaluation means for examining the subject using the data sets. The emitted directional radiation pattern utilizes its gradient of photon density to detect a hidden object while scanning across the examined subject. The wavelength of the radiation can be selected to be sensitive to endogenous or exogenous pigments, or to cause fluorescent emission from a fluorescent constituent of interest in the subject. The operation of the systems is computer controlled.

BACKGROUND OF THE INVENTION

This invention relates to examination and imaging of biological tissueusing visible or infra-red radiation.

Traditionally, potentially harmful ionizing radiation (for example,X-ray or γ-ray) has been used to image biological tissue. This radiationpropagates in the tissue on straight, ballistic tracks, i.e., scatteringof the radiation is negligible. Thus, imaging is based on evaluation ofthe absorption levels of different tissue types. For example, inroentgenography the X-ray film contains darker and lighter spots. Inmore complicated systems, such as computerized tomography (CT), across-sectional picture of human organs is created by transmitting X-rayradiation through a section of the human body at different angles and byelectronically detecting the variation in X-ray transmission. Thedetected intensity information is digitally stored in a computer whichreconstructs the X-ray absorption of the tissue at a multiplicity ofpoints located in one cross-sectional plane.

Near infra-red radiation (NIR) has been used to study non-invasively theoxygen metabolism in tissue (for example, the brain, finger, or earlobe). Using visible, NIR and infra-red (IR) radiation for medicalimaging could bring several advantages. In the NIR or IR range thecontrast factor between a tumor and a tissue is much larger than in theX-ray range. In addition, the visible to IR radiation is preferred overthe X-ray radiation since it is non-ionizing; thus, it potentiallycauses fewer side effects. However, with lower energy radiation, such asvisible or infra-red radiation, the radiation is strongly scattered andabsorbed in biological tissue, and the migration path cannot beapproximated by a straight line, making inapplicable certain aspects ofcross-sectional imaging techniques.

Recently, certain approaches to NIR imaging have been suggested. Oneapproach undertaken by Oda et al. in "Non-Invasive HemoglobinOxygenation Monitor and Computerized Tomography of NIR Spectrometry,"SPIE Vol. 1431, p. 284, 1991, utilizes NIR radiation in an analogous wayto the use of X-ray radiation in an X-ray CT. In this device, the X-raysource is replaced by three laser diodes emitting light in the NIRrange. The NIR-CT uses a set of photomultipliers to detect the light ofthe three laser diodes transmitted through the imaged tissue. Thedetected data are manipulated by a computer of the original X-ray CTscanner system in the same way as the detected X-ray data would be.

Different approaches were suggested by S. R. Arriadge et al. in"Reconstruction Methods for Infra-red Absorption Imaging," SPIE Vol.1431, p. 204, 1991; F. A. Grunbaum et al. in "Diffuse Tomography," SPIEVol. 1431, p. 232, 1991; B. Chance et al., SPIE Vol. 1431 (1991), p. 84,p. 180, and p. 264; and others who recognized the scattering aspect ofthe non-ionizing radiation and its importance in imaging. None of thosetechniques have fully satisfied all situations.

In summary, there continues to be a need for an improved imaging systemwhich utilizes visible or IR radiation of wavelengths sensitive toendogenous or exogenous pigments.

SUMMARY OF THE INVENTION

The invention relates to systems and methods for spectroscopicexamination of a subject positioned between input and detection ports ofthe spectroscopic system applied to the subject.

According to one aspect of the invention, a spectroscopic systemincludes at least one light source adapted to introduce, at multipleinput ports, electromagnetic non-ionizing radiation of a knowntime-varying pattern of photon density and of a wavelength selected tobe scattered and absorbed while migrating in the subject, the inputports being placed at selected locations on the subject to probe aselected quality of the subject; and radiation pattern control meansadapted to achieve a selected time relationship of the introducedpatterns to form resulting radiation that possesses a substantialgradient in photon density as a result of the interaction of theintroduced patterns emanating from the input ports, the radiation beingscattered and absorbed in migration paths in the subject. The gradientin photon density may be achieved by encoding the introduced radiationpatterns with a selected difference in their relative amplitude,relative phase, relative frequency or relative time. The system alsoincludes a detector adapted to detect over time, at a detection portplaced at a selected location on the subject, the radiation that hasmigrated in the subject; processing means adapted to process signals ofthe detected radiation in relation to the introduced radiation to createprocessed data indicative of the influence of the subject upon thegradient of photon density; and evaluation means adapted to examine thesubject by correlating the processed data with the locations of theinput and output ports.

Preferred embodiments of this aspect of the invention includedisplacement means adapted to move synchronously all the optical inputports or move the detection ports to another location on a predeterminedgeometric pattern; at this location the examination of the subject isperformed.

According to another aspect of the invention, a spectroscopic systemincludes at least one light source adapted to introduce, at multipleinput ports, electromagnetic non-ionizing radiation of a knowntime-varying pattern of photon density and of a wavelength selected tobe scattered and absorbed while migrating in the subject, the inputports being placed at selected locations on the subject to probe aselected quality of the subject; radiation pattern control means adaptedto achieve a selected time relationship of the introduced patterns toform resulting radiation that possesses a substantial gradient in photondensity as a result of the interaction of the introduced patternsemanating from the input ports, the radiation being scattered andabsorbed in migration paths in the subject. The system also includes adetector adapted to detect over time, at a detection port placed at aselected location on the subject, the radiation that has migrated in thesubject; displacement means adapted to move the detection port tovarious locations on a predetermined geometric pattern, the variouslocations being used to detect over time radiation that has migrated inthe subject; processing means adapted to process signals of the detectedradiation in relation to the introduced radiation to create processeddata indicative of the influence of the subject upon the gradient ofphoton density; and evaluation means adapted to examine the subject bycorrelating the processed data with the locations of the input andoutput ports.

According to another aspect of the invention, a spectroscopic systemincludes at least one light source adapted to introduce, at multipleinput ports, electromagnetic non-ionizing radiation of a knowntime-varying pattern of photon density and of a wavelength selected tobe scattered and absorbed while migrating in the subject, the inputports being placed at selected locations on the subject to probe aselected quality of the subject; radiation pattern control means adaptedto achieve a selected time relationship of the introduced patterns toform resulting radiation that possesses a substantial gradient in photondensity as a result of the interaction of the introduced patternsemanating from the input ports, the radiation being scattered andabsorbed in migration paths in the subject. The system also includes atleast one detector adapted to detect over time, at multiple detectionports placed at selected locations on the subject, the radiation thathas migrated in the subject; processing means adapted to process signalsof the detected radiation in relation to the introduced radiation tocreate processed data indicative of the influence of the subject uponthe gradient of photon density, and evaluation means adapted to examinethe subject by correlating the processed data with the locations of theinput and output ports.

Preferred embodiments of this aspect of the invention includedisplacement means adapted to move at least one of the detection portsto another location on a predetermined geometric pattern, the otherlocation being used to perform the examination of the subject.

Preferred embodiments of this aspect of the invention include rotationmeans adapted to rotate synchronously the optical input ports whileintroducing the resulting radiation along a predetermined geometricpattern, the input port rotation being used to perform the examinationof a region of the subject.

Preferred embodiments of the above described aspects of the inventionare also used to locate a fluorescent constituent of interest in thesubject; the wavelength of the introduced radiation is selected to beabsorbed in the fluorescent constituent, the detected radiation isemitted from the fluorescent constituent and processed to determinelocation of the fluorescent constituent.

According to another aspect of the invention, a spectroscopic systemincludes a light source adapted to introduce, at an input port,electromagnetic non-ionizing radiation of a known time-varying patternof photon density and of a wavelength selected to be scattered andabsorbed while migrating in the subject, the input port being placed ata selected location on the subject to probe a selected quality of thesubject; detectors adapted to detect over time, at multiple detectionports placed at selected locations on the subject, the radiation thathas migrated in the subject; the time relationship of the detection overtime, at the detection ports, being selected to observe a gradient inphoton density formed as a result of the interaction of the introducedradiation with the subject. The system also includes processing meansadapted to process signals of the detected radiation in relation to theintroduced radiation to create processed data indicative of theinfluence of the subject upon the gradient of photon density, andevaluation means adapted to examine the subject by correlating theprocessed data with the locations of the input and output ports.

Preferred embodiments of this aspect of the invention includedisplacement means adapted to move at least one of the detection portsto another location on a predetermined geometric pattern, the otherlocation being used to perform the examination of the subject.

According to another aspect of the invention, a spectroscopic systemincludes a light source adapted to introduce, at an input port,electromagnetic non-ionizing radiation of a known time-varying patternof photon density and of a wavelength selected to be scattered andabsorbed by a fluorescent constituent while migrating in the subject,the input port being placed at a selected location on the subject tolocate the fluorescent constituent of the subject; detectors adapted todetect over time, at multiple detection ports placed at selectedlocations on the subject, fluorescent radiation that has migrated in thesubject. The system also includes processing means adapted to processsignals of the detected radiation in relation to the introducedradiation to create processed data indicative of location of thefluorescent constituent of the subject, and evaluation means adapted toexamine the subject by correlating the processed data with the locationsof the input and output ports.

Preferred embodiments of this aspect of the invention includedisplacement means adapted to move at least one of the detection portsto another location on a predetermined geometric pattern, the otherlocation being used to locate the fluorescent constituent of thesubject.

Preferred embodiments of the above-described aspects of the inventionuse one or more of the following features:

The time-varying pattern comprises radiation of a selected wavelengthintensity modulated at a selected frequency. The radiation patterncontrol means are further adapted to control a selected phaserelationship between the modulated radiation patterns introduced fromeach of the input ports having to produce in at least one direction asteep phase change and a sharp minimum in the intensity of theradiation.

The radiation pattern control means are further adapted to impose on allthe introduced radiation patterns an identical time-varying phasecomponent thereby changing the spatial orientation of the direction ofthe steep phase change and the sharp minimum in the intensity of theradiation.

The time-varying pattern comprises radiation of a selected wavelengthintensity modulated at a selected frequency. The radiation patterncontrol means are further adapted to control a selected frequencyrelationship between the modulated radiation patterns introduced fromeach of the input ports having to produce in at least one direction asteep phase change and a sharp minimum in the intensity of theradiation.

The time-varying pattern comprises radiation of a selected wavelengthintensity modulated at a selected frequency. The radiation patterncontrol means are further adapted to control a selected amplituderelationship between the modulated radiation patterns introduced fromeach of the input ports having to produce in at least one direction asteep phase change and a sharp minimum in the intensity of theradiation.

The radiation pattern control means are further adapted to add to allthe introduced radiation patterns an identical time-varying amplitudecomponent thereby changing the spatial orientation of the direction ofthe steep phase change and the sharp minimum in the intensity of theradiation.

The radiation is modulated at a frequency that enables resolution of thephase shift that originates during migration of photons in the subject.

The frequency is on the order of 10⁸ Hz.

The processing means further adapted to determine the phase or theintensity of the radiation altered by scattering and absorption in thesubject.

The wavelength of the radiation is susceptible to changes in anendogenous or exogenous tissue pigment of the subject.

The gradient in photon density may also be achieved by encoding theintroduced radiation patterns with a selected difference in theirrelative amplitude, relative phase, relative frequency or relative time.

Other advantages and features of the invention will be apparent from thefollowing description of the preferred embodiment and from the claims.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1, 1A and 1B show diagrammatically phase modulation imagingsystems employing several input ports and one detection port inaccordance with the present invention.

FIG. 2 is a block diagram of the phase modulation imaging systemincluding several input ports and several detection ports in accordancewith the present invention.

FIG. 2A depicts a phased array transmitter that radiates a directionalbeam.

FIG. 2B depicts sequencing of the phases of an antiphase multi-elementarray to achieve an electronic scan of the photon density gradient inaccordance with the present invention.

FIG. 2C depicts four element antiphased array designed for a conicalscan of the photon density gradient in accordance with the presentinvention.

FIG. 2D depicts the input and output port arrangement of an imagingsystem in accordance with the present invention.

FIGS. 3 and 3A depict an imaging system for detection of a hiddenfluorescing object in accordance with the present invention.

FIG. 4 is a block diagram of an alternative embodiment of a dualwavelength PMS system.

FIG. 4A is a schematic diagram of an oscillator circuit of FIG. 4.

FIG. 4B is a schematic diagram of a PMT heterodyne modulation and mixingnetwork shown in FIG. 4.

FIG. 4C is a schematic diagram of an AGC circuit shown in FIG. 4.

FIG. 4D is a schematic diagram of a phase detector circuit shown in FIG.4.

FIGS. 5A, 5B, and 5C illustrate changes in optical field propagating ina strongly scattering medium which includes a strongly absorbingcomponent.

FIG. 6 shows an experimental arrangement of a two element phased arrayused in an interference experiment.

FIGS. 6A, 6B, and 6C show detected interference patterns of twodiffusive waves.

FIG. 7 displays the phase shifts measured for a two element array (curveA), and for a single source (curve B).

FIG. 8A depicts an experimental arrangement of sources of a four elementphased array and a detector.

FIGS. 8B and 8C display the intensities and the phase shifts measuredfor the four element array of FIG. 8A, respectively.

FIG. 9A depicts an experimental arrangement of sources of a four elementphased array, a detector, and a strongly absorbing object.

FIGS. 9B, 9C display respectively the intensities and the phase shiftsmeasured for the four element array of FIG. 9A scanning absorbingobjects of different sizes.

FIG. 9D displays the phase shifts measured for the four element array ofFIG. 9A scanning absorbing objects of different absorption coefficients.

FIG. 10A an experimental arrangement of sources of a four element phasedarray, a detector, and two strongly absorbing objects.

FIG. 10B displays the phase shifts measured for the four element arrayof FIG. 10A scanning two absorbing objects of different sizes.

FIG. 11 depict diagrammatically a single wavelength localization systemutilizing a conical scanner.

FIGS. 11A and 11B depict diagrammatically imaging systems utilizing oneor two dimensional phased array transmitters.

FIGS. 12A and 12B depict an imaging system comprising a two dimensionalphased array transmitter and detection array.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Imaging system embodiments of the present invention based uponinterference effects of radiation migrating in a subject havingscattering and absorptive properties are shown in FIGS. 1, 2, and 3. Thesystems effectively utilize, in this scattering medium, a directionalbeam of visible or IR radiation generated and/or detected by an array ofsources and/or detectors, respectively. For instance, in the case of anarray of sources, each source is placed at a selected location in thearray and emits intensity modulated radiation, preferably coherentradiation from a laser diode, of a selected intensity and phase. Thecriteria for selecting the source locations, the intensities, and thephases of the respective sources is the shape of the desired beam thatat any time point possesses a substantial photon density gradientproduced by interference effects of radiation from the various sources.This gradient of photon density is localized and has directionalproperties. Overall, the resulting radiation formed by interference ofthe radiation of the individual sources migrates in a selected directionin the subject. In an antiphase system, the wavefront of the beam hassections of equal photon density separated by a sharp localized changein photon density. Selected different locations of the photon densitygradient are shown in FIG. 2B.

In general, the wavefront propagates in the selected direction in thesubject and the gradient of photon density is localized in one or moreplanes extending from the source array in a selected direction. If thesubject includes a localized object having different scattering andabsorptive properties from those of the surrounding environment, thepropagating radiated field is perturbed. This perturbation is detectedand from the source detector geometry the perturbing object can belocated.

Referring to the embodiment of FIGS. 1 and 1A, the imaging systemutilizes an array of laser diodes 12, 14, 16, and 18 for introducinglight into the tissue at selected locations. The geometry of opticalinput ports 11, 13, 15, 17 and of an optical output port 19 is selectedto examine a specific part of the tissue. From the known geometry of theoptical input ports and the detection port and from the shape of theintroduced and detected radiation, a computer can locate a hidden object9 of examined tissue 8 (for example, a head or breast). A masteroscillator 22, which operates at 200 MHz, excites laser diodes 12through 18, that emit light of a selected wavelength (e.g., 760 nm). Thelight from each laser diode is conducted to the respective input portplaced on a subject via a set optical fibers. A detector 24 detects thelight that has migrated through the examined tissue. Preferably,detector 24 includes a photomultiplier tube (e.g., Hamamatsu R928)powered by a high voltage supply which outputs about 900 V in order toensure a high gain. A local oscillator 26 operating at a convenientoffset frequency (e.g., 25 kHz) sends a signal to a mixer 28 and areference signal to detector 24. Accordingly, an output waveform 25 fromdetector 24 is at a carrier frequency equal to the difference of thedetected and reference frequency, i.e., 25 kHz.

Detector 24 (for example, PMT Hamamatsu R928 or Hamamatsu R1645u)detects the scattered and absorbed light that has migrated through thesubject. Detection port 19 is located several centimeters from thelocation of the input ports. The PMT detector is connected to thesubject by the fiber optic guide, or, alternatively, may be directlyplaced on the subject. It has been found that the most cost-effectivedetector for measuring signals of frequencies on the order of 10⁸ Hz isHamamatsu R928. However, the Hamamatsu R1645u detector is preferred dueto its high precision. The second dynode of the PMT of detector 24 ismodulated by 200.025 MHz signal 27 so that the 25 kHz hetrodyned signal25 is received by a phase detector 30. Phase detector 30 also receivesreference signal 29 from mixer 28. If phase detector 30 is a lock-inamplifier then the output signals are the phase shift and the intensityof the detected signal. Both the phase shift and the intensity of thedetected light characterize the migration path of photons in the subject(e.g., the brain tissue).

Alternatively, a tunable dye laser or other laser source connected to awide band acousto-optical modulator operating at the carrier frequency,e.g., 200 MHz can be used instead of the laser diode. Theacousto-optical modulator modulates the intensity of the light emittedby the laser at the selected carrier frequency.

The invention also envisions using only one source of coherent lightthat irradiates one end of several optical fibers at the same time. Theother end of each fiber is placed on the subject at a selected inputport location. This source radiates light of a selected time varyingpattern. The phase relationship and the intensity of the light carriedby each fiber is varied by creating a time delay (e.g., different fiberlength) and by coupling different amounts of light into each fiber.

FIG. 1B shows diagrammatically an imaging system of FIG. 1 furtheradapted to encode the transmitted light using an offset frequency.Oscillators 22a, 22b, 22c and 22d drive four laser diodes at frequencies30.025 MHz, 30.035 MHz, 30.045 MHz and 30.055 MHz, respectively. Thelaser diodes introduce the light that migrates in tissue 8 and iscollected at detection port 19 and detected by PMT detector 24. Localoscillator 26 provides a 30 MHz reference signal to detector 24 thatoutputs a detection signal having 25 kHz, 35 kHz, 45 kHz and 55 kHzfrequency components. Each component signal is phase detected at acorresponding phase detector (30a, 30b, 30c and 30d) having a suitablefrequency filter. The phase detectors provide a phase shift, migrationpathlength and amplitude for each frequency.

The imaging systems of FIGS. 1, 2, and 3 are shown to have a lightsource of a single wavelength; however, a dual wavelength imaging systemis also envisioned according to this invention. In the dual wavelengthimaging system two laser diodes or a tunable wavelength laser generatelight of two wavelengths that is coupled to an optical fiber. Such asystem will now be described.

A dual wavelength operation is shown in FIG. 4. The system includes amaster oscillator 60 operating at 200 MHz and an oscillator 62 operatingat 200.025 MHz which is offset 25 kHz from the master oscillatorfrequency. The offset frequency of 25 kHz is a convenient frequency forphase detection in this system; however, other offset frequencies ashigh as a few megahertz can be used. Oscillator 60 alternatively drivestwo sets of laser diodes 64a, 64b, . . . , 64n and 66a, 66b, . . . , 66nusing switches 61a, 61b, . . . , 66n. These switches are drivenelectronically to couple a selected wavelength into the optical fiberand also to achieve a selected radiation pattern resulting from theradiation emanating from the individual fibers. An output 8 mm fibercoupler 72 collects photons for an R928 PMT detector 74. The seconddynode (shown in FIG. 3B) of PMT 74 is modulated with a 200.025 MHzreference signal generated by oscillator 62 and amplified by anamplifier 63. Thus, the output signal of the PMT detector has afrequency of 25 kHz. PMT detector 74 alternately detects light of thetwo laser diodes that has migrated in the tissue and producescorresponding output signals, which are filtered by a filter 76 andleveled by an automatic gain control (AGC) circuit 79. A referencesignal of 25 kHz is produced in a mixer 65 by mixing the 200 and 200.025MHz oscillator signals. The reference 25 kHz signal is also leveledusing the second AGC 77 and fed into a phase detector 79. Phase detector79 generates a signal indicative of the phase of each output signalrelative to the phase of the reference signal. The outputs of phasedetector 79 are alternately selected by an electronic switch 80,filtered, and then input to an adder 82 and a subtractor 81 to producesum and difference signals proportional to <L>.sub.λ1 +<L>.sub.λ2 and<L>.sub.λ1 -<L>λ₂. The difference and sum signals are then used tocalculate changes in the probed pigment and in the blood volume,respectively.

A schematic diagram of preferred oscillator 60 or 62 is shown in FIG.4A. This circuit has a drift of only 0.03 degrees/hr. (Weng, et al.,"Measurement of Biological Tissue Metabolism Using Phase ModulationSpectroscopic Measurement," SPIE, Vol. 143, p. 161, 1991, which isincorporated herein by reference). The crystal is neutralized, whichenables operation at resonance, and thus achieves long-term stability.The respective crystals of oscillators 60 and 62 are offset from eachother by 25 kHz. This circuit provides a sufficient output to directlydrive a 5 mW laser diode.

A modulation circuit 75 for the second dynode of the PMT is shown inFIG. 4B. This circuit uses a resonant circuit 75a with an impedance of20,000 ohms instead of the usual 50 Ω load with very high powerdissipation, providing a 50 V drive of the photomultiplier dynode whiledissipating only a few watts of power.

To obtain stable operation of the phase detector, a stable input signalis required. The 25 kHz AGC circuit 77, 78 illustrated in FIG. 4Cincludes an MC 1350 integrated circuit U1, featuring wide range AGC foruse as an amplifier. The signal amplitude is controlled by a feedbacknetwork, as shown. A major reason for the accurate detection of phasechanges by the PMT system is that the phase detector input signal levelis kept nearly constant by the AGC circuit. Since the input voltagechange of between 2 and 6 volts causes variation in the phase shift ofonly 0.2%, the AGC circuit eliminates the need for a very stable highvoltage power supply.

A preferred phase detector circuit is shown in FIG. 4D. Two sinusoidalsignals (the measurement signal and the reference signal) aretransformed to a square wave signal by a Schmitt trigger circuit 79a.The phase of the square wave signal is shifted by an RC change (composedof R11, R12, C8), which makes it possible to change the measuring range.The detector further includes a 74HC221 integrated circuit. The lock-inamplifier technique obtained to derive the difference of the phase andamplitude of the two signals has the highest signal to noise ratiopossible for this type of equipment.

The above-described systems utilize the carrier frequency on the orderof 10⁸ Hz which is sufficiently fast to resolve the phase shift of thedetected light. The characteristic time, the time it takes for a photonto migrate between an input port and an output port, is severalnanoseconds. The sensitivity of the system is high, approximately 70°per nanosecond or 3° per centimeter change of pathlength, as observed inexperimental models. Selection of the modulation frequency also dependson the desired penetration depth and resolution of the imaging systemthat will be described below. If deep penetration is desired, a lowmodulation frequency (e.g., 40 MHz) is selected, and if shallowpenetration is needed, modulation frequencies as high as 10⁹ Hz can beused.

Referring to FIGS. 1 and 1A, a master oscillator 22 operates at amodulation frequency in the range of 40 to 400 MHz selected according tothe desired penetration depth of the optical field. The array of laserdiodes 12, 14, 16, and 18 generates a highly directional radiationpattern, which is employed in the tissue examination.

In one preferred mode of operation, laser diodes 12 to 18 operate in aphased array pattern which is introduced into the tissue and detected bya single PMT detector 30. Master oscillator 22 operating at 200 MHzdrives a multi-channel phased splitter which gives outputs atpredetermined phases. Input ports 11 through 17 are located at selecteddistances and an appropriate phasing of the array creates a directionalbeam and enables scanning of the optical field in two dimensions acrossthe tissue, as shown in FIGS. 2A, 2B, and 2D. After migrating throughthe tissue, the optical field is collected in a large area fiber onselected locations 19. The detected signals are heterodyned in the PMTdetector 24 by utilizing the output of local oscillator 26, operating ata 25 kHz offset frequency, to detector 24. The resulting 25 kHz signalis phase detected with respect to the output signal 29 of mixer 28 anddetector 24. Phase detector 30 outputs the phase and the intensity ofsignal 25. The detected phase shifts and intensities are stored and usedfor construction of an image of the subject. This is performed bycomputer control 34, which governs the operation of the system.

FIG. 2 depicts a phase modulation imaging system comprising an inputport array for introducing radiation and a detection port array fordetecting radiation that has migrated in the subject. The operation ofthe system is controlled by computer control 34, which coordinates atransmitter unit 32 with a receiver unit 42. Transmitter unit 32comprises several sources of visible or IR radiation adapted tointroduce a selected time-varying pattern of photon density into subject8 by array of input ports 31, 33, 35, and 37. Receiver unit 42 detectsradiation that has migrated in the subject from the input port array toan array of detectors 39, 41, 42, and 47.

The radiation sources of transmitter unit 32 are intensity modulated ata frequency in the range of 40 MHz to 200 MHz, as described for theimaging system of FIG. 1. Receiver unit 42 detects and processes theradiation using the same principles of the phase and amplitude detectionas described above. The signal detected at individual ports can bephased using appropriate delays.

Several modes of operation of the transmitter array and receiver arrayare described in FIGS. 2A, 2B, 2C, and 2D. Referring to FIG. 2A, it hasbeen known, that for a simple horizontal linear array of N identicalelements radiating amplitude modulated light spaced a distance, d,apart. The radiating wavefront is created by the interference effect. Ifall elements radiate in phase the wavefront propagates in a directionperpendicular to the array. However, by appropriately phasing theradiating elements, the resulting beam can scan space in two dimensions.We consider the phases of the signal along the plane A--A whose normalmakes an angle θ₀ with respect to the array normal. The phase of thesignal from the first radiator lags the phase of the second radiator bya phase angle (2π/λ)d sin θ₀ because the signal from the second radiatorhas to travel a distance d sin θ₀ longer than the signal from the firstradiator to reach plane A--A. Similarly, the phase of the signal fromthe nth radiator leads that from the first radiator by an anglen(2π/λ))d sin θ₀. Thus, the signals from the various radiators can beadjusted to be in-phase along the A--A plane, if the phase of eachradiator is increased by (2π/λ)d sin θ₀. Consequently, at a point on thewavefront in the far field of the transmitter array the signals from theN radiators will add up in phase, i.e., the intensity of the totalnormalized signal is a sum of the signals from the individual sources.The constructed pattern has a well defined directional characteristicand a well pronounced angular dependence, i.e., the transmitter patternhas a well defined transfer characteristic of the transmitter withrespect to the angle θ₀.

FIG. 2B depicts an arrangement of phases for the sources the system ofFIG. 2 operating in one preferred mode of operation. The array of fivesources is divided into two or more portions that are phased 180° apart.Each portion has at least one source. The sources of each portionradiate amplitude modulated light of equal intensity and are spaced sothat the resulting beam of two or more equally phased sources has asubstantially flat wavefront, i.e., no gradient of photon density. Onthe other hand, there is a sharp 180° phase transition, a large gradientin photon density between two antiphased portions of the array. Thus,the radiated field possesses an amplitude null and a phase transition of180° (i.e. crossover phase), which is due to the large gradient ofphoton density.

Electronic scanning is performed by appropriately varying theapportionment of 0° and 180° phases on the sources. The five elementarray of FIG. 2B can have the 180° phase transition along four differentparallel planes extending from the array. Scanning is achieved byelectronically switching the sources by 180°, so that the photon densitygradient moves in the direction parallel to the location of the sources.

Using the principles described in FIGS. 2A and 2B, a conical scan of adirectional beam possessing at least one substantial photon densitygradient can be accomplished using a four element antiphased array, asshown in FIG. 2C. The laser diodes are antiphased using a push pulltransformer. The phasing and amplitude of four laser diodes S₁, S₂, S₃,and S₄ arranged into a two dimensional array is modified sequentiallyusing the switches Sw₁, Sw₂, Sw₃, and Sw₆ and inductances L₁, L₂, L₃,and L₄.

FIG. 2D shows a possible arrangement of the transmitter array and thereceiver array. The above described directional beam enters subject 8 atthe transmitter array location and is pointed to hidden absorber 9 whichperturbs the migrating beam. The field perturbation is measured by thereceiver array. Scanning of the transmitter array or the receiver arrayis envisioned by the present invention.

A hidden absorber that includes a fluorescent constituent is detectedusing a selected excitation wavelength of the laser sources of thetransmitter array. Then, the radiation is absorbed, and almost instantlya fluorescent radiation of a different wavelength is re-emitted. There-emitted radiation propagating in all directions is detected by thereceiver array.

FIG. 3 depicts a phase modulation imaging system comprising one inputport and several arrays of detection ports. This system operatescomparably to the systems of FIGS. 1 and 2. The 754 nm light of a laserdiode 48 is amplitude modulated using master oscillator 22. The light iscoupled to subject 8 using an input port 49. The amplitude modulatedlight migrates in the subject and is scattered from hidden object 9. Itis also expected that hidden object 9 has a different effective index ofrefraction than subject 8. The migrating radiation is governed by thelaws of diffusional wave optics that are described below. The scatteredradiation migrates in several directions and is detected by detectionsystems 50, 52, and 54.

Ports 51, 53, and 55 of the detection systems can include either largearea fibers or arrays of detection ports. If large area fibers are usedthen detector systems 50, 52, and 54 correspond to detector 24 ofFIG. 1. If arrays detection ports are used, then each of detectorsystems 50, 52, and 54 includes several individual PMT detectors. ThePMT detectors of each detector system are phased utilizing a selectedphase mode, as described above. The phasing is controlled by thecomputer control. The detected signals are heterodyned at the PMTdetectors and sent to a phase detector 58. Phase detector 58 detectsalternatively the heterodyned signals using a switch 56. Operation ofphase detector 58 is similar to the operation of phase detector 30 ofFIG. 1. The detected phase and amplitude are alternatively sent to thecomputer control using a switch 56a. Even though only one phase detectoris shown in FIG. 3, the invention envisions use of several phasedetectors.

If hidden absorber 9 includes a fluorescent constituent, laser diode 48is selected to introduce an excitation wavelength (e.g., 754 nm). Theintroduced, intensity modulated radiation, excites the fluorescentconstituent which re-emits radiation in all directions, as shown in FIG.3. The re-emitted radiation is detected using detector systems 50, 52,and 54. To increase the system resolution, each detector can befurnished with an interference filter selected to pass only thefluorescent radiation.

FIG. 3A shows diagrammatically an imaging system used for detection of afluorescing object 9. The system is a modified version of the system ofFIG. 3 wherein a four element phase array 47 introduces a 200 MHz lightof a 0° and 180° phase. The diffusion wave emitted from array 47 isre-emitted by object 9 and detected by ports 51, 53 and 55 and processedas described in connection with FIG. 3. Array 47 effectively codes theillumination light. Thus, when array 47 is rotated about the examinedorgan with object 9, the receivers will contain informationcorresponding to the orientation of the object. Each detection port alsoincludes a filter that passes only the fluorescent radiation; thisimproves the resolution of the system.

The interference of several waves, as described in FIG. 2A, has beenlong known in a non-scattering medium, wherein the radiation propagateson a straight line, but not in a strongly scattering medium. Referringto FIGS. 6, 6A, 6B, and 6C, in a simple experiment, interference of twodifferent diffusive waves in a strongly scattering medium wasdemonstrated. Propagation of visible IR radiation in a scattering mediumsuch as tissue can be described by diffusion of photons, and thus wedescribe it as a diffusive wave that exhibit refraction, diffraction andinterference. The diffusive waves, which can be visualized as "ripplesof brightness," represent a scalar, over-damped traveling waves of lightenergy density.

Referring to FIG. 6, the two laser diodes were separated at a distanceof 4 cm and 1.2 cm from the detection port. The intensity modulatedlight of the two laser diodes at frequency 200 MHz was sent through twooptical fibers to a container with an Intralipid™ suspension. The sourcedetector distance was varied by moving the optical port of the detectionfiber along a line parallel to the position of the sources. FIGS. 6A,6B, and 6C show measured maxima and minima of the optical fieldmigrating in the medium. This data demonstrates interference between twodiffusive waves created by two coherent emitting sources of phasedifference 180 degrees. FIG. 7 summarizes the experiment, wherein thedisplacement of the detector is plotted against the phase shift measuredby the detector. The phase shift displays the steepest part of thetrace, curve A, (slope of about 360°/cm) at the displacement of about2.25 cm. Curve B is measured with an optical field of source. S₂. Here,the measured slope is about 30°/cm. When comparing curves A and B wedemonstrate much higher sensitivity of the null detection of the twoelement array contrasted with a diminished sensitivity to the detectordisplacement when using a single source arrangement. The sensitivity ofthe two source arrangement is increased by about a factor of 10. Thesensitivity is further increased when using four or more element phasedarray, which sharpens the photon density gradient and thus provides ahigher resolution for locating the hidden object.

In a strongly scattering medium, the emitted photons undergo a largenumber of collisions and their migration can be determined by applyingthe diffusion equation. The diffusion equation for photons in auniformly scattering medium was solved by E. Gratton et al., "Thepossibility of a near infrared optical imaging system using frequencydomain methods." in Mind Brain Imaging Program, Japan 1990; and by J.Fishkin et al., "Diffusion of intensity modulated near-infrared light inturbid media", SPIE Vol. 1413 (1991) p. 122. A solution of the diffusionequation was obtained for the light of a point source (at r=0) radiatingS{1+M exp -i(ωt+e)!} photons, wherein S is the source strength(photons/sec.), M is the modulation of the source at frequency ω, and eis an arbitrary phase. The photon intensity can be calculated as

    I(r,t)=c*ρ(r,t),

wherein ρ(r,t) is the photon density and c=10⁸ m/s is the velocity oflight.

When solving the diffusion equation using a spherical-harmonicsapproximation in a non-absorbing medium for the density of photonsρ(r,t) than

    I(r,t)=(I.sub.0 /Dr)+(I.sub.0 /Dr) exp  -r(ω/2cD).sup.1/2 !×exp  ir(ω/2cD).sup.1/2 -i(ωt+e)!,

wherein the diffusion constant D is 1/3 of the mean free path. In theabsence of an amplitude modulated signal (ω=0) the solution correspondsto a spherical wave propagating without attenuation. For a non-zerofrequency, the amplitude of the signal at a frequency ω decreasesexponentially. The light wave front the emitted advances at the constantvelocity V

    V=(2Dcω).sup.1/2

and has wavelength

    λ=2π(2cD/ω).sup.1/2

The above equations show that higher modulation frequencies yieldshorter effective wavelengths, and smaller diffusion constants also giveshorter effective wavelengths. In principle, short wavelengths can beobtained using high frequency modulated waves in a very turbid medium.However, the amplitude of the modulated wave decreases exponentiallywith the modulation frequency. Therefore, the best resolution, i.e., theshortest wavelength, is obtained using the highest frequency which stillgives a measurable signal. The diffusion process limits the penetrationdepth at any given modulation frequency, because of the exponentialdecrease of the wave's amplitude, and also decreases the velocity oflight propagation.

The above described diffusion wave approach treats amplitude modulatedlight waves in scattering media using the framework of wave optics. Thephoton intensity, calculated as superposition of different waves,constitutes a scalar field, propagating at a constant velocity. At anygiven modulation frequency, the wave optics phenomenology of scalarfields is valid. Therefore, in the frequency-domain, the measurement andanalysis of light diffusing in tissues from several sources will undergoconstructive and destructive interference. Furthermore, wave refractionoccurs at a boundary between two different tissues. It causes adeviation of the direction of propagation of the wave front, and thusthere is a change in the amplitude and phase shift of the propagationwave. The direction change is a function of the ratio of the effectiveindex of refraction in the two tissues. In diffusional wave optics, onthe other hand, the wave's amplitude is exponentially attenuated as thewave propagates in the scattering medium. This attenuation is inaddition to the exponential attenuation caused by finite absorption ofthe medium.

Amplitude modulated waves propagate coherently in the scattering medium;this is crucial for image reconstruction. It is possible to accuratelymeasure in real time, the average intensity, amplitude, and phase of thewave front over a large area using a single detector or an array ofdetectors applying well-established frequency-domain methods.

The emitters are varied sequentially in phase starting with the firstemitter in the line and followed by subsequent emitters. Each emitteremits a spherical wave and propagation of the resultant beam isperpendicular to the wavefront. If all the transmitter delays are equal,the beam travels straight ahead. Delay lines which produce variabletransmitter delays can be used to obtain appropriate phasing forsteering the beam across the tissue. The same principle can apply duringreception.

There are two important aspects of imaging as envisioned by the presentinvention. The first is a geometrical aspect and the second is phasingof the transmitters and receivers.

It is also possible to construct a two-dimensional array fortwo-dimensional pointing (e.g., FIG. 2C). The multiplexing switches usedwith these arrays can be constructed as an integral part of the arrayand can consist of field effect transistors arranged so that access toany element may be obtained by the application of two adverse signals.

In addition to electronic scanning, the two-dimensional scanning can beachieved by moving the array of sources and detectors in a regularpre-determined pattern in a plane parallel to that being investigated inthe subject. For maximum detection, the detector is places in the planeof the photon density gradient of the resulting field created by thearray of sources. The plane of the photon density gradient is swept asthe array moves. In this sweeping action, as a strongly or weaklyabsorbing object enters the radiation field, the detector registers afield imbalance due to the above described refraction of the propagatingradiation. A two-dimensional image is formed by storing the informationwhile the probe is moved across the subject. Several scans in differentimaging planes are envisioned by the invention. If the system isduplicated or time shared in two other faces of a cube, an algorithmwould be used to provide a 3-dimensional picture of the object bytriangulation. For a linear array of sources, there is a plane in whichthe null is sensitively detected, and the intersection of three planes(particularly at orthogonal intersection) defines the location of ahidden absorber. The data storage is accomplished electronically.

The detector detects the intensity and the phase shift of the radiationthat has migrated in the subject. The phase shift depends on the tissueproperties, i.e., absorption and scattering. For the low frequencies thephase shift is proportional to ((1-g)μ_(s) /μ_(a))^(1/2) and for thehigh frequencies proportional to 1/μ_(a). To obtain desired penetrationdepth, appropriate frequency for both master oscillator 22 and localoscillator 26 is chosen; however, the phase relationship of the laserdiodes is maintained.

Different types of phased arrays are designed for optimal examinationand imaging of different human organs (e.g., human head or breast). Forexample, a mosaic of optical input ports and optical detection portsdefined by positions of optical fibers attached to a skull cap may beused. A standardized mapping may be developed also using x-raytechniques. Contrast labeling of different physiological structures willaid the visualization and orientation. The amplitude and phase of thesignals can be monitored on a precision oscilloscope. In order to scanthe phased array past a fixed object of approximately known position, asin needle localization procedures, the location of the input and outputports will be scanned past the object and the position of maximum phaseshift will be recorded in one-dimension; however, detection in two andthree dimension can be performed in the same way.

In the preferred mode of operation, the array of sources is phased 180°apart, as shown in FIG. 8A. There is a sharp 180° transition of photondensity wave, a large gradient in photon density, from S₂, S₂ sources tothe S₃, S₄ sources. Thus, the radiated field gives an amplitude null anda phase transition of 180° corresponding to the y-z plane, i.e.,perpendicular to the detector. If a larger number of similarly phasedsources is used, the transitions are even sharper. The array produces auniform photon density pattern on each side of the array, as shown inFIGS. 8B and 8C. If an absorbing object is placed in this directionalfield of diffusing optical waves, imbalance in the photon density ismeasured. The detection of a hidden object is accomplished bytranslating the experimental transmitter-receiver system of FIG. 8A.

In addition to the mechanical scanning achieved by moving of theinput-output port system, electronic scanning can be performed using themultiple source and multiple detector system of FIG. 2. As shown in FIG.2B for an array of five sources, there is a 180° phase transition in theresulting migrating field due to the 180° phase difference between theantiphased sources radiating amplitude modulated light. The plane of the180° phase transition can be shifted in parallel by appropriatelyvarying the apportionment of 0° and 180° phases on the sources. This isperformed by sequentially switching the phase of the sources by 180°. Ineach case, the detection port located on this plane is used forcollecting the data. As the sources are electronically switched by 180°,the detection array can be also electronically switched from onedetection port to another. The signal from the receiving optical fiberis coupled to one shared PMT detector. However, the system can alsoinclude several detectors. If the systems of FIGS. 1 or 1A are used, theelectronic source scanning can be combined with synchronous mechanicalmovement of the detection port.

In general, the invention utilizes the photon density gradient createdin the migrating field since it increases the resolution of thedetection. As known to one skilled in the art, the photon densitygradient formed by interference effects of introduced waves can becreated not only by appropriate phasing of the sources but also by othermethods such as appropriately spacing the sources, creating an imbalancein the radiated intensity of the individual sources, and other. Theimbalance may be achieved by modulating the amplitude of one source withrespect to another; this displaces the null in the correspondingdirection. Furthermore, the introduced signal can be encoded by thefrequency or a selected phase.

FIG. 8A shows the arrangement of the input ports 11 to 17 and detectionport 19 of FIG. 1. As described above, light of each laser diode 12through 18 is intensity modulated at the 200 MHz frequency. Wavelengthof the intensity modulated radiation is ##EQU1## wherein f is themodulation frequency of 200 MHz, μ_(s) is the scattering factor which isapproximately 10 cm⁻¹ in an Intralipid solution with refractive index n,and c is 3×10⁸ cm/s. Thus, the expected wavelength is about 7 cm. Theinput ports S₁, S₂, S₃, and S₄ are set 3.5 cm apart and are anti-phasedby 180° using a push pull transformer. The antiphased array creates alarge gradient in photon density chosen to take advantage of thedestructive interference with the null detection. The laser diodesemitting 754 nm light are intensity modulated at 200 MHz using masteroscillator 22, and the local oscillator 26 is operating at 200.025 MHzto perform the dynode modulation of PMT detector 24. The detectedintensities and phase shifts of an x-direction scan (FIG. 8A) ofdetection port 19 are plotted in FIGS. 8B and 8C, respectively. Asexpected, the intensity has a sharp minimum in between sources S₂ and S₃where the phase is changed 180°. The peak width at half maximum is about2 cm. In addition to the x-direction scan of the detection port, thedetection port was scanned in y-direction wherein, as expected, novariation was observed.

Referring to FIG. 9A, cylindrical objects of different diameter, d, werescanned using the previously described phased array. The objects wereplaced in the middle of the linear array displaced 2.5 cm from thex-axis. The detection port was located on the x-axis and each object wasmoved parallel to the x-axis at the 2.5 cm y displacement. The intensityand phase shift detected at different locations are plotted in FIGS. 9Band 9C, respectively. The intensity pattern for each moving object hastwo maximum and one minimum when the scanned object was located at x=0,y=2.5 point during its scan along the x-axis. At this point, a largephase change is detected, as shown in FIG. 9C. The phase detection hasinherently larger resolution of a localized absorber; a hidden object ofsize as small as 0.8 mm can be detected.

The response due to different absorption of the hidden object wasstudied using a 5 mm cylinder of different absorption coefficientscanned by the 4 element phased array of FIG. 9A. The detected phasechange is shown in FIG. 9D. The 5 mm black rod displays the largestphase change due to its high absorption, and the cylinder filled withcardiogreen 3.5 mg/l which has absorption coefficient μ_(a) =200 cm⁻¹shows the smallest phase change. In scanning of a hidden object, theseexperiments correspond to mechanically displacing the source detectorsystem, or electronically scanning the subject.

Scanning of two objects of a different diameter is shown in FIG. 10A.Two cylinders of different diameter are scanned across the four elementphased array located on the x-axis. The detection port is located at y=5cm. In FIG. 10B the detected phase change is plotted against thedisplacement of these objects. Curve A represents the phase change oftwo cylinders of diameters 5 mm and 10 mm separated 3 cm apart. Curve Bwas measured using 16 mm cylinder instead of the 5 mm cylinder. In thiscase, wherein the two cylinder separation is smaller, the phase detectorcan not resolve the two objects.

The imaging resolution is increased by increasing the number of elementsof the phased array, since the main lobe of the resultant beam becomesmuch sharper, the gradient of photon density is larger. Phased arrays ofdifferent number of elements and different shapes are used for imagingdifferent organs. For example, in tumor imaging, the four element phasedarray of FIG. 8A having an approximately linear shape can be used forimaging of the brain. On the other hand, a rectangular or a circularphased array would be used for imaging of a hidden tumor in the breast.The modulation frequency and the element spacing is adjusted to obtainproper focussing in each case.

In general, an imaging system will operate using the following modes ofoperation that arise from the above-described principles. In the firstmode of operation, a series of zero phased, appropriately spaced sourcescreate photon diffusion waves. One or more detectors sensitive to aselected wavelength detect the phase and the amplitude of the migratingwave. Individual sources and detectors may be coded and activatedaccording to selected detection and display schemes. The second mode ofoperation uses a series of sources phased at 0° and 180° (or any otheroffset phase that gives adequate sensitivity) with respect to eachother. The detector set at the null point of the array detects changesin the phase at the null point. Each detector may use an interferencefilter to limit its sensitivity to a selected wavelength. The third modeof operation may further complement the second mode by not onlydetecting the phase transition but also the amplitude null. The mostsensitive detection is achieved when a hidden object is located in themidline plane of the 0°-180° signal. An object is located using bothsignals and their appropriate integrals or derivatives are used toenhance the resolution of the system. The display will also utilizeinformation from several wavelengths, for example, when 750 nm and 850nm sources are used, the signal difference provides information aboutthe hemoglobin oxygenation and the sum about the blood concentration.Other wavelengths sensitive to endogenous or exogenous tissue pigmentsmay be used. The same source array may be designed to operate in allthree modes of operation. A computer supervisory system selects asuitable mode of operation for optimal sensitivity.

Referring to FIG. 11, a single wavelength localization system 83 employsa conical scanner 85 that introduces optical radiation of a selectedwavelength from four laser sources 87 to tissue 8. The relationship ofthe introduced patters is selected so that the resulting introducedradiation pattern forms a cone scanning in the examination space. Theoperation principles of array 87 were described in connection with FIGS.2A, 2B and 2C. Oscillator 62 generates a 200.025 MHz drive signal 91that is introduced to modulators 90a and 90b. Furthermore, the phase ofthe drive signal is shifted by 90° in modulator 90a relative to thephase of the drive signal is modulator 90b, and the phase signals arevaried over time at 60 Hz. Each of the quadrature phase signals (92, 93)are splitted in splitter 89a and 89b to from an in-phase and anti-phasedrive signals. The four drive signals drive four laser diodes labeled N,S, W and E of array 87. Thus array 87 generates a scanning conicalsignal (88) that includes a sharp phase change in the center of thesignal cone. Array 87 has four 780 nm laser diodes, but otherwavelengths selected for a high sensitivity to a tissue component may beemployed. Furthermore a multi-wavelength array can also be used.

The introduced diffusive photon density wave migrates in tissue 8 and isdetected at optical port 86 of an optical fiber connected to PMTdetector 75. As described above, the detected radiation is heterodynedusing a 200 MHz reference signal and the corresponding 25 kHz signal iscoupled to amplitude detector 96 and phase detector 79. Phase detector79 measures the phase shift between the introduced and detectedradiation patterns. The output of the phase detector is correlated withthe 60 Hz signals 92 and 93 to produce localization signalscorresponding to the N, S, W and E laser sources. The localizationsignals may be monitored using an oscilloscope.

When port 86 is symmetrically arranged in respect to the location of theradiation cone 88 and there is no field perturbation (i.e., no hiddenobject 9), the oscilloscope will display a circular pattern. In the samearrangement of cone 88 and port 86, if hidden object 9 is located in theradiation field, the oscilloscope pattern will no longer be symmetrical,e.g., the circular pattern may change to an elliptical pattern. Formaximum sensitivity, detection port 86 mechanically scans around tissue8 and is locked onto the scanning conical signal so that port 86 alwayspoints to the center of cone 88, i.e., port 86 is in the null location.

Referring to FIG. 11A, a phase modulation imaging system 100 includes atwo-dimensional phased array transmitter 102 connected to laser sources104. Electronics 120 drives laser sources 104 and also providesreference signals to the detection system. Optical detector 150 includesan optical input port 152 defined by a relatively large area opticalfiber 154 connected to a PMT detector 156.

Phased array transmitter 102 includes a horizontal array 106 and avertical array 112 of input ports connected by a set of optical fibers(not shown in FIG. 11) to laser sources 104 that include 754 nm and 816nm laser diodes labeled a and b, respectively. Diodes 107, 108, 109, and110 of the horizontal array 106 are driven by a push-pull transformer122, and diodes 103, 114, 115, and 116 of the vertical array 112 aredriven by a push-pull transformer 124. The resolution of the system maybe increased by adding more sources.

The horizontal sources are intensity modulated at a frequency ofapproximately 200.025 MHz generated by 200.025 MHz oscillator 124 and ahorizontal TV scan drive 128 generating a saw-tooth signal of 60 Hz. Ahorizontal reference signal 127 of 25 kHz supplied to phase detector 162is produced in a mixer 126 by mixing the 200.025 MHz signal fromoscillator 124 and a 200 MHz signal from oscillator 121. The verticalsources are intensity modulated at a frequency of approximately 200.2MHz generated by a 200.2 MHz oscillator 134 and a vertical TV scan drive138 generating a saw-tooth signal 139 of 1 kHz. A vertical referencesignal 137 of 200 kHz supplied to phase detector 164 is produced in amixer 136 by mixing the 200.2 MHz signal from oscillator 134 and the 200MHz from oscillator 121.

The emitted light of either 754 nm or 816 nm, alternated at 60 Hz by achopper, migrates in the examined tissue as described above and isdetected at input port 152. The detected light is heterodyned at PMTdetector 156 receiving a reference 200 MHz signal from oscillator 121.The detector signal is then filtered at 25 kHz and 200 kHz using filters158 and 160, respectively. Phase detectors 162 and 164, receiving 25 kHzand 200 kHz reference signals, respectively, determine at each frequencythe phase shift of the detected light in respect to the introducedlight.

As described above, the phase shift and the related optical pathlengthof the migrating photons directly reflect the tissue properties. System100 can distinguish the differences in the phase shift of the lightemitted from horizontal array 106 and vertical array 112 since theemitted light from each array is modulated at a slightly differentfrequency.

Transmitter array 102 is designed to reflect the geometry of theexamined tissue and a possible location of hidden objects. The hiddenobjects A, B, and C of FIG. 11 targeted by array 102 are 3 to 4 cm inthe scattering medium. Thus, array 102 has the input ports spacedapproximately 1 cm apart and equidistantly from the center. Detectionport 152 is located about 7 to 10 cm from transmitter 102 and may bemechanically scanned in correlation with the total introduced field ofarray 102.

PMT detector 156 receives signals from the horizontal and verticalarrays. The modulation offset vertical frequency of the waveform isabout 10 times higher than for the horizontal waveform since therepeatability of the vertical scan is higher than the repeatability ofthe horizontal scan. Approximately the same frequency difference is usedfor the horizontal and vertical TV scans. The output from phasedetectors 162 or 164 represents the phase value as detected along thehorizontal axis and the vertical axis. A localized absorbing orscattering object (e.g., a tumor, localized bleeding) will cause a"resonance curve" type response. The detected phase shifts for eachsignal is differentiated (166 and 168) to "sharpen" the chances andincrease the resolution. The horizontal and vertical outputs are addedin a summing amplifier 170 and are coupled to a video input of a 500line TV display 180. The display may be graded in a gray scale or afalse color scale. The resolution achieved in the above describedone-dimensional experiments can be further improved and thesignal-to-noise ratio enhanced by employing a computer storage of thescanned data and integrating over a number of scans and using contrastenhancing algorithms. Alteratively, a "slow scan" TV may be used withnarrow banding of the outputs of the phase detectors.

System 100 may also include an amplitude detector 157 that detects theamplitude of the detected radiation at the 25 kHz and 200 kHzfrequencies. The detected amplitude signals are manipulated the same wayas the phase shift signals and fed to display 180. The use of both theamplitude signals and the phase signals improves resolution of theimage.

FIG. 11B shows diagrammatically a low frequency imaging system 190 thatemploys techniques similar to the ones used in system 100 of FIG. 11A. Asource array 192 emits diffused waves that propagate in tissue 195 andare detected by detectors 200. The highest resolution is achieved when ahidden object is located on the null line of the diffused waves. Thesystem operates at about 50 MHz, to use instead of the laser diodesLED's and instead of the PMT detectors Si diodes. Oscillators 202 and204 drive phase splitters 206 and 208, respectively, that provide twointensity modulated voltage signals shifted 180° with respect to eachother. The 0° and 180° signals drive 750 nm and 850 nm, LED sourceswhich are multiplexed by switches 210 and 212 to operate sources of onewavelength at the same time. The modulated diffuse waves are detected bythe Si diodes that include a wavelength specific interference filter,and the detector signal are converted from 50 MHz and 50.01 MHzfrequencies to 20 kHz frequencies using mixers 226 and 228,respectively. Phase detectors 230 and 232 operating at 20 kHz determinethe phase shift of the detected signals. Both the phase shift signalsand the amplitude signals are used to image the hidden absorber on adisplay unit 240.

A 2-dimensional transmitter and receiver arrays are shown in FIG. 12A.The spacing of the input ports can be varied depending on the frequencyof operation, expected location of the hidden object and the shape ofthe examined organ. FIG. 12B shows diagrammatically an imaging systemutilizing the 2-dimensional transmitter and receiver arrays 250 and 255that can be switched on electronically. A master oscillator 262 and alaser driver 260 drive a pair of in-phase and anti-phase laser diodes,e.g., the first and the third diode of Y-array and Z-array. A set ofelectronic switches is used to connect a different set of laser diodesevery 10 msec. A set of optical fibers transmits the detected light to aPMT detector 264 that also receives a reference 200.025 MHz signal froma local oscillator 266.

The heterodyned resulting signal is sent to a phase detector 272 thatmeasures the phase shift of the detected radiation. The measured phaseshift is further manipulated to enhance the detected changes on a CRTdisplay 276 which has the same 10 msec time base as electronic switches263. Differenciator 274 takes a derivative of the phase shift signal;this intensifies the crossover of the phase shift shown in FIGS. 8c, 9cand 9d.

Alternative Embodiments

In addition to the above described directional detection, the presentinvention envisions imaging systems constructed to calculate the averagemigration pathlengths. Referring to FIG. 4, in such system the drivesignal from oscillator 60 is introduced to a selected laser diode 64a, .. . , 64n or 66a, . . . , 66n using switches 61a, . . . , 61n. Theintensity modulated radiation of each laser diode is coupled to tissue70 at an input port located at a precisely defined position. A detectionport located at another position detects radiation that has migrated intissue 70. The detected signal is heterodyne mixed directly at PMTdetector 74. These signals are fed into the phase detector wherein thephase and the intensity of the detected radiation are measured. Thesystem may include several PMT detectors and phase detectors (only oneset of detectors is shown in FIG. 4) operating simultaneously or onedetector scans the surface of tissue 70. The phase shift and theintensity of the detected heterodyned signal depend on the tissuethrough which said scattered and absorbed radiation migrated.

The tissue properties are determined from the detected phase shift andintensity values and from the known input ports and detection portgeometries. The measured average pathlengths, <L>, can also bedetermined. The detected phase shift is converted to an effectivemigration pathlength <L> by using the low frequency approximationθ=2πf<L>n/c, wherein f is the modulation frequency, c is the speed oflight (3×10⁸ cm/s), and n is the refractive index of the medium.

To illustrate imaging by detecting migration pathlengths, we use anexample of photon migration in a tissue with a strongly absorbingobject, a perfect absorber(μ_(a) →∞) of radius R. Referring to FIGS. 5A,5B, and 5C the distribution of pathlengths defines an optical field thatexists between a point detector, D, and source, S, separated by distanceρ and located on the exterior of an examined tissue which is asemi-infinite, strongly scattering medium. As shown in FIG. 5A,infinitely far away from the field, a perfect absorber does not alterthe banana-shaped optical field of photons emitted by source S anddetected at detector D. As the object enters the optical field (FIG.5B), the photons which have migrated the farthest distance from D and Sare eliminated by the absorption process inside the perfect absorber ofradius R. Since photons which travel the longest pathlengths areabsorbed, the approach of an object shortens the distribution ofpathlengths, or alternatively, shortens the average pathlength <L>. Asthe object moves closer, and the optical field surrounds the object(FIG. 5C), some of the detected photons have travelled "around" theobject, which is detected as lengthening the distribution ofpathlengths. Thus, the average pathlength measurement can reveallocation of a strongly absorbing component of a tissue (e.g., tumor orlocalized bleeding).

Even though this pathlength computation approach requires in most casesextensive computational capabilities, it can yield useful information inthe localization procedures and can provide a useful supplement to theabove described directional approach.

What is claimed is:
 1. A spectroscopic system for examination ofbiological tissue comprising:at least one light source, operativelyconnected to at least one oscillator, constructed to generate opticalradiation of a visible or infrared wavelength, said radiation beingintensity modulated at a selected frequency of said oscillator toachieve a known radiation pattern, multiple input ports, placed atselected locations relative to the examined tissue, optically connectedto said at least one light source and constructed to introducesimultaneously optical radiation to the examined tissue at thecorresponding tissue locations, a radiation pattern controllerconstructed to control the radiation intensity or phase relationship ofpatterns simultaneously introduced from said multiple input ports, saidintroduced patterns forming resulting radiation that possesses asubstantial gradient of photon density in at least one direction, saidresulting radiation being scattered and absorbed in migration paths inthe examined tissue, a detection port placed at a selected locationrelative to the examined tissue, an optical detector, opticallyconnected to said detection port, constructed and arranged to detectover time the resulting radiation that has migrated in the tissue tosaid detection port, a processor, operatively connected to and receivingsignal from said detector, arranged to process signals of said detectedradiation in relation to said introduced radiation to create processeddata indicative of influence of the examined tissue upon saidsubstantial gradient of photon density of said resulting radiation, andan imager, including a display, connected to and receiving saidprocessed data from said processor, said imager being constructed toproduce image data representing the influence of the examined tissueupon said substantial gradient of photon density over a multiplicity ofdetection locations and to display said image data on said display. 2.The system of claim 1 wherein said multiple input ports are arranged toform a one dimensional array shaped to examine a selected organ of ahuman body.
 3. The system of claim 1 wherein said multiple input portsare arranged to form a two dimensional array is shaped to examine aselected organ of a human body.
 4. The system of claim 2 or 3 whereinsaid shape fits a head or a female breast.
 5. The system of claim 1wherein said wavelength is sensitive to a wavelength sensitive agentpresent in the examined tissue and said processed data also indicate ofthe influence of said wavelength sensitive agent upon said steep phasechange or said sharp minimum in the intensity of said radiation.
 6. Thesystem of claim 1 further comprising a phase detector, connected to andreceiving signals from said optical detector, constructed to detect thephase of the detected radiation, said phase being utilized by saidimager.
 7. The system of claim 1 further comprising an amplitudedetector, connected to and receiving signals from said optical detector,constructed to detect the amplitude of the detected radiation, saidamplitude being utilized by said imager.
 8. The system of claim 1further comprising a second light source, optically connected to saidinput ports, constructed to generate radiation of a second selectedwavelength in the visible or infrared range.
 9. The system of claim 1further comprising:a second detection port placed at a selected locationrelative to the examined tissue, an second optical detector, opticallyconnected to said second detection port, constructed and arranged todetect over time the resulting radiation that has migrated in the tissueto said detection port, and said processor, receiving signal from saidfirst detector and said second detector, further arranged to processsignals of said first detector and said second detector in a relativephase shifted relationship to create said processed data.
 10. The systemof claim 9 wherein said relative phase shifted relationship comprises a180 degree phase shift between said first detector signal and saidsecond detector signal.
 11. A spectroscopic system for examination ofbiological tissue comprising:at least one light source, operativelyconnected to at least one oscillator, constructed to generate opticalradiation of a visible or infrared wavelength, said radiation beingintensity modulated at a selected frequency of said oscillator toachieve a known radiation pattern, multiple input ports, placed atselected locations relative to the examined tissue, optically connectedto said at least one light source and constructed to introducesimultaneously optical radiation to the examined tissue at thecorresponding tissue locations, a radiation pattern controllerconstructed to control the radiation intensity or phase relationship ofpatterns simultaneously introduced from said multiple input ports, saidintroduced patterns forming resulting radiation that possesses asubstantial gradient of photon density in at least one direction, saidresulting radiation being scattered and absorbed in migration paths inthe examined tissue, a detection port placed at a selected locationrelative to the examined tissue, an optical detector, opticallyconnected to said detection port, constructed and arranged to detectover time the resulting radiation that has migrated in the tissue tosaid detection port, a processor, operatively connected to and receivingsignal from said detector, arranged to process signals of said detectedradiation in relation to said introduced radiation to createlocalization signals indicative of influence of the examined tissue uponsaid substantial gradient of photon density of said radiation, and animager, including a display, connected to and receiving saidlocalization signals from said processor and being constructed todisplay said localization signals indicating scattering or absorption ofthe examined tissue over a multiplicity of detection locations.
 12. Thesystem of claim 11 wherein said processor further comprises a phasedetector constructed to detect the phase of the detected radiation. 13.The system of claim 11 wherein said processor further comprises anamplitude detector constructed to detect the amplitude of the detectedradiation.
 14. The system of claim 1, or 11 wherein said phaserelationship of radiation patterns introduced from two adjacent inputports of said multiple input ports is 180 degrees.
 15. The system ofclaim 11 further comprising a second light source, optically connectedto said input ports, constructed to generate radiation of a secondselected wavelength in the visible or infrared range.
 16. The system ofclaim 15 further constructed to introduce said radiation of saidwavelengths in a time-shared manner.
 17. The system of claim 11 whereinsaid multiple input ports are arranged to form a one dimensional arrayshaped to examine a selected tissue region of a human body.
 18. Thesystem of claim 11 wherein said multiple input ports are arranged toform a two dimensional array is shaped to examine a selected tissueregion of a human body.
 19. The system of claim 17 or 18 wherein saidshape fits a head or a female breast.
 20. The system of claim 1 or 11further comprising multiple detection ports optically connected to atleast one said detector and placed at selected locations relative to theexamined tissue, said detection ports forming a one dimensional array ora two dimensional array.
 21. The system of claim 1 or 11 wherein saidprocessor further calculates from said processed data an absorptioncoefficient μ_(a) or a scattering coefficient μ_(s).
 22. The system ofclaim 11 wherein said resulting radiation forms a cone scanning in theexamined tissue and said localization signals include a non-symmetricalpattern indicating presence of an abnormal tissue region of differentscattering or absorption properties.
 23. The system of claim 1 or 11wherein said input and detection ports are arranged in a transmissiongeometry.
 24. The system of claim 1 or 11 wherein said wavelength issensitive to a wavelength sensitive agent present in the examined tissueand said processed data also indicate of the influence of saidwavelength sensitive agent upon said steep phase change or said sharpminimum in the intensity of said radiation.
 25. The system of claim 11wherein said imager further employs a contrast enhancing algorithm onsaid localization signals.
 26. The system of claim 11 wherein saidimager further employs a contrast enhancing algorithm that includesdifferentiation of the localization signals.
 27. The system of claim 11wherein said imager further employs a contrast enhancing algorithm thatincludes integration of the localization signals.
 28. The system ofclaim 1 or 11 wherein said radiation pattern controller is constructedto alter the relative phase, the relative amplitude or the relativefrequency of said introduced radiation patterns to achieve electronicscanning of said gradient of photon density of said resulting radiationover the examined tissue.
 29. The system of claim 1 or 11 furthercomprising a mechanical scanning device constructed to move said inputports to different locations on a predetermined geometric patternrelative to the examined tissue.
 30. The system of claim 1 or 11 furthercomprising a mechanical scanning device constructed to move saiddetection port to a different location on a predetermined geometricpattern relative to the examined tissue.
 31. The system of claim 1 or 11wherein said oscillator operates at a frequency in the range of 40 MHzto 1 GHz.
 32. The system of claim 1 or 11 wherein said oscillatoroperates at a frequency of about 50 MHz.
 33. The system of claim 32wherein said light source is a LED diode and said detector is a diodedetector.
 34. The system of claim 1 or 11 wherein said oscillatoroperates at a frequency of about 200 MHz.
 35. The system of claim 34wherein said light source is a laser diode and said detector is a PMT.36. The system of claim 1 or 11 wherein said substantial gradient ofphoton density includes a steep phase change or a sharp minimum in theintensity of said resulting radiation.
 37. The system of claim 1 or 11wherein said wavelength of said light source is 754 nm, 760 nm, 780 nm,800 nm, 816 nm, 840 nm or 850 nm.
 38. The system of claim 1 or 11wherein said oscillator is an element of a double side band modulatorconstructed to produce said intensity modulation.
 39. The system ofclaim 1 or 11 wherein said detector is a photomultiplier constructed forheterodyne detection.
 40. A spectroscopic method of examination ofbiological tissue of a subject, comprising the steps:providing at leastone light source of electromagnetic radiation of a visible or infraredwavelength, said radiation being intensity modulated at a selectedfrequency, said source being optically connected to multiple inputports, and further providing a detection port optically connected to anoptical detector; introducing simultaneously into the examined tissue,at said input ports, said intensity modulated radiation having a knowntime-varying pattern of photon density, a time relationship, radiationintensity or phase of said patterns being selected by a patterncontroller to form resulting introduced radiation that possesses, in atleast one direction, a substantial gradient in photon density as aresult of interaction of the simultaneously introduced patternsemanating from said input ports, said resulting radiation beingscattered and absorbed in migration paths in the examined tissue;detecting over time, at a detection port placed at a selected locationon the examined tissue, said radiation that has migrated in the examinedtissue; processing signals of said detected radiation in relation tosaid introduced radiation to create processed data indicative ofinfluence of said examined tissue upon said substantial gradient ofphoton density; and imaging the examined tissue by displaying image datarepresenting the influence of the examined tissue upon said substantialgradient of photon density over a multiplicity of detection locations.41. The method of claim 40 further comprising the steps ofmovingsynchronously said input ports or said detection port to differentlocations on a predetermined geometric pattern relative to the examinedtissue; introducing into the examined tissue, at each said input port,said radiation; detecting over time, at said detection port saidradiation that has migrated in the examined tissue; processing signalsof said detected radiation in relation to said introduced radiation tocreate processed data indicative of the influence of said examinedtissue upon said gradient of photon density; and imaging the examinedtissue of said different location by displaying image data created fromsaid processed data received from said processor.
 42. The method ofclaim 40 further comprising the steps ofintroducing said radiation whileelectronically scanning said gradient of photon density of saidresulting radiation over the examined tissue; detecting over time, atsaid detection port said radiation that has migrated in the examinedtissue; processing signals of said detected radiation in relation tosaid introduced radiation to create processed data indicative of theinfluence of said examined tissue upon said gradient of photon density;and imaging the examined tissue of said different location by displayingimage data created from said processed data received from saidprocessor.
 43. The method of claim 42 wherein said electronic scanningstep is performed by varying the relative phase, the relative amplitudeor the relative frequency of said introduced radiation patterns.
 44. Themethod of claim 40, 41 or 42 further comprising the step of positioningsaid detection port in the direction of said gradient of photon density.45. The method of claim 40, 41 or 42 further comprising the step ofdetecting the phase of said detected radiation and utilizing said phasein said imaging.
 46. The method of claim 40, 41 or 42 further comprisingthe step of detecting the amplitude of said detected radiation andutilizing said amplitude in said imaging.
 47. The method of claim 40, 41or 42 further comprising the steps of introducing, at each said inputport, said radiation of a second selected wavelength in the visible orinfrared range;detecting over time, at said detection port saidradiation of said second selected wavelength that has migrated in theexamined tissue; processing signals of said detected radiation of saidsecond selected wavelength in relation to said introduced radiation ofsaid second selected wavelength to create processed data indicative ofthe influence of said examined tissue upon said gradient of photondensity; and imaging the examined tissue by displaying image datacreated from said processed data received from said processor.
 48. Themethod of claim 40 further comprising the steps of:detecting over time,at multiple detection ports placed at selected locations relative to theexamined tissue, said radiation that has migrated in the examinedtissue, said detection ports forming a one dimensional array or a twodimensional array; processing signals of said detected radiation inrelation to said introduced radiation to create processed data ofmultiple detection ports indicative of the influence of the examinedtissue upon said gradient of photon density; and imaging the examinedtissue by displaying said image data.
 49. The method of claim 40 furthercomprising the steps of:detecting over time, at two detection portsplaced at selected locations relative to the examined tissue, saidradiation that has migrated in the examined tissue, processing signalsdetected at said first detection port and said second detection port ina relative phase shifted relationship to create said processed data. 50.The method of claim 49 wherein said relative phase shifted relationshipcomprises a 180 degree phase shift between the signal detected at saidfirst detection port and the signal detected at said second detectionport.
 51. A spectroscopic method of examination of biological tissue ofa subject, comprising the steps:providing at least one light source ofelectromagnetic radiation of a visible or infrared wavelength, saidradiation being intensity modulated at a selected frequency, said lightsource being optically connected to multiple input ports, and furtherproviding a detection port optically connected to an optical detector;introducing simultaneously into the examined tissue, at each said inputport, said intensity modulated radiation of a known time-varying patternof photon density, a relative time relationship, radiation intensity orphase of said introduced patterns being selected by a pattern controllerto form resulting radiation that possesses, in at least one direction, asubstantial gradient in photon density as a result of interaction of thesimultaneously introduced patterns emanating from said input ports, saidresulting radiation being scattered and absorbed in migration paths inthe examined tissue; detecting over time, at a detection port placed ata selected location relative to the examined tissue, said radiation thathas migrated in the examined tissue; processing signals of said detectedradiation in relation to said introduced radiation to createlocalization signals indicative of influence of the examined tissue uponsaid substantial gradient of photon density; and imaging the examinedtissue by displaying said localization signals indicating scattering orabsorption of the examined tissue over a multiplicity of detectionlocations.
 52. The method of claim 51 further comprising the steps ofmoving synchronously said input ports or said detection port to adifferent location on a predetermined geometric pattern relative to theexamined tissue;introducing into the examined tissue, at each said inputport, said radiation; detecting over time, at said detection port saidradiation that has migrated in the examined tissue; processing signalsof said detected radiation in relation to said introduced radiation tocreate localization signals indicative of the influence of said examinedtissue upon said gradient of photon density; and imaging the examinedtissue by displaying said localization signals of said locationindicating scattering or absorption of the examined tissue.
 53. Themethod of claim 51 further comprising the steps of introducing saidradiation while electronically scanning said gradient of photon densityof said resulting radiation over the examined tissue;detecting overtime, at said detection port said radiation that has migrated in theexamined tissue; processing signals of said detected radiation inrelation to said introduced radiation to create localization signalsindicative of the influence of said examined tissue, by said electronicscanning, upon said gradient of photon density; and imaging the examinedtissue by displaying said localization signals indicating scattering orabsorption of the examined tissue.
 54. The method of claim 53 whereinsaid electronic scanning step is performed by varying the relativephase, the relative amplitude or the relative frequency of saidintroduced radiation patterns.
 55. The method of claim 51, 52 or 53further comprising locating said detection port in the direction of saidgradient of photon density.
 56. The method of claim 51, 52 or 53 furthercomprising the step of detecting the phase of said detected radiationand utilizing said phase in said step of producing said localizationsignals.
 57. The method of claim 51, 52 or 53 further comprising thestep of detecting the amplitude of said detected radiation and utilizingsaid amplitude in said step of producing said localization signals. 58.The method of claim 56 further comprising the step of calculating fromsaid processed data an absorption coefficient μ_(a) or a scatteringcoefficient μ_(s).
 59. The method of claim 40 or 51 wherein the phaserelationship of radiation patterns introduced from two adjacent inputports of said multiple input ports is 180 degrees.
 60. The method ofclaim 40 or 51 wherein said introducing and detecting steps areperformed having said input and detection ports arranged in atransmission geometry.
 61. The method of claim 60 wherein said imagingstep further comprises the step of employing a contrast enhancingalgorithm on said localization signals.
 62. The method of claim 60wherein said imaging step further comprises the step of employing acontrast enhancing algorithm that includes differentiation of thelocalization signals.
 63. The method of claim 60 wherein said imagingstep further comprises the step of employing a contrast enhancingalgorithm that includes integration of the localization signals.
 64. Themethod of claim 40 or 51 further comprising the step of varyingfrequency of the introduced radiation to alter migration path of saidresulting radiation in the examined tissue.
 65. The method of claim 40or 51 wherein said frequency of the introduced radiation is in the rangeof 40 MHz to 1 GHz.
 66. The method of claim 40 or 51 wherein saidfrequency is about 50 MHz or about 200 MHz.
 67. The method of claim 40or 51 wherein said substantial gradient of photon density includes asteep phase change or a sharp minimum in the intensity of saidradiation.
 68. The method of claim 40 or 51 wherein said wavelength ofsaid light source is sensitive to a wavelength sensitive agent presentin the examined tissue.
 69. The method of claim 40 or 51 wherein saidwavelength of said light source is 754 nm, 760 nm, 780 nm, 800 nm, 816nm, 840 nm or 850 nm.
 70. The method of claim 40 or 51 wherein saidwavelength of said light source is sensitive to an endogenous or anexogenous pigment.
 71. The method of claim 40 or 51 wherein saidintensity modulation is performed by a double side band modulator. 72.The method of claim 40 or 51 wherein said detector is a photomultiplierand said detecting step includes heterodyne detection.
 73. The method ofclaim 40 or 51 further comprising the steps of introducing radiation ofa second wavelength, said radiation being intensity modulated at saidfrequency and having said known time-varying pattern of photon density.74. The method of claim 73 wherein said steps of introducing saidradiation of said wavelengths is performed in a time-shared manner.